1. Field of the Invention
The present invention relates to machinery and processes for manufacturing spaced apart thermal seals in applications such as pouch-type bags in which metered quantities of substances, such as food stuffs, are sealed therein.
2. Description of the Prior Art
The Assignee of the present invention manufactures machinery for making thermally sealed pouch-type bags. Machines of this type are well known. A continuous web having the front and back sides of the bag printed thereon is folded in half and run over the outside periphery of a first sealing system to which the present invention relates in part. The sealing system has evenly spaced thermally actuated sealing elements mounted on the periphery of a rotating wheel which forms parallel spaced apart seals by melting a thermoplastic material coated on the inside of the web after the web has been folded in half. After the parallel seals are made by the sealing system, each bag in the web has been formed except for the closure of the top. After running across the sealing system, the continuous web with parallel spaced apart seals which define the opening of each bag is fed to a pouch filling station at which a metered quantity of substance, such as powered food, is placed inside the individual bags located along the continuous web as the bags at the pouch filling station rotate around the periphery thereof. After filling with the metered quantity of substance, the bags then pass to a second thermal sealing station which heat seals the top of each bag to close the food stuff or other material therein in a sanitary manner. Finally, the sealed bags, which are still part of the continuous web, pass to a cutting station at which a series of rotating knives cut the individual bags apart uniformly in the middle of the parallel spaced apart thermal seals which were formed by the first sealing system.
The Assignee of the present invention manufactures machines of the aforementioned type which operate at high processing speeds, such as up to 4000 linear inches per minute or higher, to make individual pouch-type bags from the moving web. High speed operation in machinery of this type is extremely important as a consequence of the economics of manufacturing products such as dry packaged foodstuffs. Any circumstance occurring during manufacturing of filled thermally sealed pouch-type bags which requires the line to be shut down has a serious effect on the productivity of the manufacturing operation.
Materials from which thermally sealed pouches are manufactured include paper, cellophane foil, metallized structures, polypropylene and other laminates of preprinted or unprinted stock.
In the prior art, two basic types of sealing systems, which typically operate with the axis of rotation of the sealing wheel of the sealing system being vertical, are known. These sealing systems are known as “drum” and “modular” sealing systems.
The drum style sealing system is a single piece of metal machined from a casting which has a calculated land diameter that is the heated surface contacting the moving web to form the spaced apart parallel seals. Drum style sealing systems have the disadvantage of not having an adjustable spacing between the parallel seals which define the width of the pouch-type bag. Behind each land and parallel thereto is a hole containing a tubular electric heater element. Drum style sealers work well once the substantial thermal mass of the machined casting has come up to operating temperature which is typically a temperature of 280° F. or above. The drum, as a consequence of its high thermal mass, will produce unacceptable seals if the continuously moving web has to be stopped for any reason. This high thermal energy of the drum at the point of contact of the lands with the web for a substantial period of time either causes the parallel seals to be destroyed because of the prolonged contact or the parallel seal grow to a greater width than desired which restricts the size of the bag. A reduced volume of the bag caused by wider seals either prevents the desired metered quantity of material from being placed inside the bag or worse, the spacing between the bags at the cutting station becomes out of synchronism with the spacing of the cutting knives causing severing of the web at spaced apart locations which do not correlate with the middle of the thermally formed seals which can cause the bags to have faulty seals or spill the contents thereof.
The modular sealing system is illustrated in FIG. 1. The modular style sealing system 10 includes a wheel having a stationary lower circular plate 12 and an upper rotating annular ring 14 to which individual seal forming elements 16 are mounted by threaded type connectors 17. The seal forming elements make peripheral contact with a continuous web 18 as illustrated in FIG. 2. The wheel of the sealing system 10 is driven by rotating shaft 18. Cooling air is blown from holes 19. The individual seal forming elements 16 may be moved radially in slots 20 to permit adjustment of the spacing between the parallel seals 24 by loosening and tightening of the connectors 17. Each of the sealing elements 16, like the aforementioned drum style sealer wheel, has a tubular electric heating element (not illustrated) mounted in a longitudinally extending hole in each seal forming element 16.
The modular style sealing system 10 is a substantial improvement over the drum style sealing system by permitting adjustability of the spacing between individual spaced apart seals 24 on the continuously moving web 18. The overall thermal mass of the sealing elements 16 results in a substantial variation in the temperature along the longitudinal dimension thereof. The modular style sealing system typically operates with a temperature differential along the longitudinal dimension between the top and bottom of the individual sealing elements 16 of 30° to 40° F. and between 30° to 50° F. between different sealing elements 16. In part, the difference in temperature along the longitudinal dimension of the sealing elements 16 during operation is caused by heat flow from the electrical heating element of the individual sealing elements 16 to the top rotating annular plate 14 during rotation and stoppage thereafter. In order to obtain a high quality seal across the length of a pouch-type bag, it is necessary to operate the modular sealing wheel at a temperature which, at its lowest point is a temperature such as 280° F., which can result in some of the individual sealing elements 16 operating at the center thereof, which are more thermally insulated than the top and bottom portions of the sealing wheel, at a temperature of at least 320° F.
As illustrated in FIG. 2, during normal operation, the continuous web 18 rolls across the periphery of the individual sealing elements 16. The sealing elements 16 have precisely chosen spacings therebetween so that each peripherally outwardly facing surface 22 contacts the continuous web 18 at the same place as the web moves around the periphery of the wheel. The result of the precise registration during moving of the continuous web 18 in contact with the outer surfaces 22 of each sealing element 16 is the formation of a straight uniform width seal 24 even though there is the aforementioned temperature variation of 30 to 40° F. during operation between the center of the individual heating elements and the ends thereof. Because of the high speed rotation of the web across the sealing wheel 10, there is a controlled short dwell time which produces uniformity in seal width, even though the aforementioned temperature variation exists.
However, if, for any reason the continuous movement of the web 18 has to be stopped, the higher operating temperature of the midpoint of the sealing elements 16, which have substantial thermal mass, causes the individual surfaces 22 to contact the continuous web 18 sufficiently long to produce a seal 26 which grows in width toward the midpoint. The wider seal 26, produced as a result of the longer contact time of the exterior surfaces 22 with the web 18 during stoppage, produces individual bags having an hour glass type interior shape which reduces the capacity of the bag to be filled. After the machine is again started, seals of the shape 26, when they pass to the filling station of the pouch-type bags, often prevent the precisely metered quantity of material, which is normally placed in the bags during continuous operation, from being placed in the individual pouch-type bags. As a result, the volume of material sealed in the bags is reduced and may vary. This can cause spillage of material because it is left over from the precisely metered quantity which is made available to be placed into the pouch-type bag and tends to fall from the elevated filling station. This is wasteful and further can require costly cleanup operations. Finally, the thermally sealed bags, as a result of having less material therein than the desired amount, may not register precisely with the downstream cutting knives at the cutting station which can result in the continuous web being severed at points which are not at the midpoint of the individual seals 26, resulting in total loss of the material.
Pouches having the hourglass interior, as illustrated in FIG. 2, are much more likely to cause jams in the feeding of the web 18, may cause stoppage of the processing and even breakage of the web. Rethreading of the web is a time consuming and costly process when stoppage of the manufacturing of individual pouches is taken into consideration. Finally, even if there is no appreciable spillage at the station which fills the bags and if the bags are properly severed, downstream quality control should reject the individual bags as not containing the correct weight of material.
The modular style sealer systems also have a large thermal mass which requires approximately 8-10 minutes to heat the sealer to the set point temperature at which thermal sealing is performed. This heat up time can result in a loss of 8,000-10,000 pouches given that machinery of this type is designed to operate at speeds of 1,000 pouches or higher per minute. Furthermore, when stoppage occurs, as illustrated in FIG. 2, it takes several minutes for the sealing system to cool down so that the web 18 will not be burned or produce the widened seals 26 which, in the past, in some applications, have required the blowing of cooling air from holes 19 in the lower circular plate 12 and is a significant cost to the manufacturer of pouch-type bags.